IMMUNITY AND ANAPHYLAXIS 191 



dose (M.L.D.) of toxin for one guinea-pig of 250 grm. weight 

 and which kills it within 5 days ; the serum and toxin 

 being mixed together and made up to 4 c.c. bulk, and 

 injected subcutaneously, the animal survives the time- 

 limit. A serum containing one such unit in 1 c.c. is 

 called "normal serum" or "normal diphtheria antitoxin" 

 (D.A.N.), while a serum 1 c.c. = 1,000 M.L.D. is spoken of 

 as ten times normal (D.A.N.) 10 , etc. One c.c. of the normal 

 serum is said to contain one " immunization unit." Work- 

 ing back from quantities of serum of known strength (anti- 

 toxic) and preserved in a dried state in a vacuum and in 

 a dark cool place, the potency of any toxin at hand can be 

 determined, and against this latter, any newly prepared 

 serum can be standardized. In this way a fairly uniform 

 standard can be maintained. The usual antitoxic serum 

 on the market contains 2000 " immunity units " or shortly, 

 units in 4 to 5 c.c. of serum and equal to 200,000 M.L.D. 

 High-potency sera are prepared so that 5 c.c. contain 5000 

 units, and correspondingly. The serum keeps very well 

 for at least one year in a cool dark place. The durability 

 of the serum is tested by keeping back some of the bottles, 

 and from time to time examining their activity, and if it is 

 found to rapidly diminish, all the bottles bearing the same 

 number are recalled. 



OTHER IMMUNITY PHENOMENA. 



Pfeiffer's Phenomenon. In 1894, Pfeiffer showed 

 that when cholera spirilla are injected into the peritoneal 

 cavity of cholera-immune guinea-pigs, the spirilla rapidly 

 swell up, become granular, and often undergo complete 

 solution. The same result could be observed in a normal 

 animal, if a protective amount of cholera-immune serum 

 were injected at the same time. The constituents of the 

 blood serum which cause this result are spoken of as 

 " Bacteriolysins." It was soon shown that the same 

 result followed the mixing of the spirilla and the serum in a 

 test tube under suitable conditions. The same phenomenon 

 was thereafter observed for other micro-organisms. 



Agglutination. In 1896, Grueber and Durham, 

 investigating Pfeiffer's phenomenon, found that when a 



